It is often necessary in semiconductor processing to fill a high aspect ratio gap with insulating material. One example of a high aspect ratio trench encountered in semi-conductor processing is in the formation of shallow trench isolation (STI). As device dimensions shrink and thermal budgets are reduced, void-free filling of high aspect ratio spaces (AR>3.0:1) becomes increasingly difficult due to limitations of existing deposition processes. The deposition of doped or undoped silicon dioxide assisted by high density plasma CVD, a directional (bottom-up) CVD process, is the method currently used for high aspect ratio (AR) gap-fill. Evolving semiconductor device designs and dramatically reduced feature sizes have resulted in several applications where HDP processes are challenged in filling the high aspect ratio structures (AR>7:1) using existing technology (see, for example, U.S. Pat. No. 6,030,881). For structures representative of the 65 nm and 45 nm technology nodes, engineering the gap-fill process becomes structure dependent, hence the process needs to be reoptimized, a task of considerable complexity, every time a new structure needs to be filled.
An alternative to CVD is atomic layer deposition (ALD). ALD methods involve self-limiting adsorption of reactant gases and can provide thin, conformal dielectric films within high aspect ratio features. An ALD-based dielectric deposition technique typically involves adsorbing a metal containing precursor onto the substrate surface, then, in a second procedure, introducing a silicon oxide precursor gas. The silicon oxide precursor gas reacts with the adsorbed metal precursor to form a thin film of metal-doped silicon oxide. One drawback, however, to ALD is that the deposition rates are very low. Films produced by ALD are also very thin (i.e., about one monolayer); therefore, numerous ALD cycles must be repeated to adequately fill a gap feature. These processes are unacceptably slow in some applications in the manufacturing environment.
A related technique, referred to as rapid vapor deposition (RVD) processing, is another alternative. RVD is similar to ALD in that reactant gases are introduced alternately over the substrate surface, but in RVD the silicon oxide film can grow more thickly. Thus, RVD methods allow for rapid film growth similar to using CVD methods but with the film conformality of ALD methods. The process is described in the paper by Hausmann et. al. entitled Rapid Vapor Deposition of Highly Conformal Silica Nanolaminates (2002, Science, 298, pages 403-406).
The polymeric film deposited by the above process requires densification in order for its properties to match those of silicon oxide (USG), which will allow of its succesful integration into functioning devices. Moreover, the conformal nature of the process results in the formation of seams in filled trenches, which may allow attack by post gap fill wet etch (HF-based) in the seam. Etching in the seam can allow for polysilicon deposition in the seam during subsequent processing which would obviate its insulating effect. Therefore, a process sequence to anneal the film and substantially eliminate seams and voids is required.
Optimally, this process sequence can operate at under 500° C., in order to be able to meet the thermal budget requirements of advanced devices (see A. R. Londergan, et. al., J. Electrochem. Soc., vol. 148, pp. C21-C27, January 2001). In pre-metal dielectric (PMD) applications, for example, where a layer of silica is applied over gates that have been previously built-up on a substrate, there is an inherent temperature limitation due to the material used to construct the gates (usually a metal silicide such as a nickel silicide.) At temperatures above about 400-500° C., the gate silicide may become discontinuous, thereby increasing the resistance of the circuit and leading to performance problems. Further, there may be two or three PMD layers applied to a substrate before application of metal conductors. Thus, it is desirable to implement a method to improve the material properties of the dielectric film while maintaining temperatures that are low enough to avoid damaging underlying structures.